Be aware there may be key changes, and there are loads of potential scales to base their key off. These are the one I encounter the most.

So the basic idea is the take your list of chords, and figure out how they all fit into one of the above patterns. Once you get that you can read off what scale you are in.

There are all kind of reasons why using the melody itself may not be the best approach. Often it makes songs interesting to use modes and other tricks to play notes that shouldn't strictly fit. I'm not going to go into that now as that may be unecessary. As a bassist it's far more useful to have the chords, work out the key (or keys if it changes) and then go from there. Of course, some chords will fit into multiple keys.

This sort of practise is incredibly useful when improvising or writing your own material. Being able to take a set of chords and work out the key is something that is essential. By the sounds of it, looking at some interval ear training may be useful too - being able to tell the sound of a major 3rd or minor 3rd is useful.

Eventually, by listening to alot of songs it will get easier. What song was it you had in mind? If it is well known there will be tabs of it on the internet, so for the first few use those to help you. There's a few good books on guitar music theory too by Hal Leonard I would recommend if you want to get into this sort of thing.

Disclaimer - as music is very subjective there are almost always exceptions to rules. If you're looking at jazz or something similar then some crazy stuff will happen.

I think he's trying to invent a new song. So he just has a fleeting image of the tune in his head.
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luser droogJul 31 '13 at 5:04

nope, I want play an existing song in Guitar, and feel finding Chords for song is much difficult than finding notes :(
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Charan PaiJul 31 '13 at 6:44

I'm not suggesting you spend ages trying to figure out the chords for yourself - I would have no chance of doing that if you gave me a song I knew well. Until you get to the point where you can make an educated guess of the key just from listening I would look the chords up online. The Ultimate Guitar website has a huge library of songs, so chances are you can find yours in there. If you want to work out the melodic line for yourself then you will at least be able to determine the key centre, and therefore what notes are likely to be used. It's a similar process to writing your own melody.
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FolauJul 31 '13 at 9:14

It looks like you're trying to figure out how to play a song by listening to it.

Unfortunately, there are few hard and fast rules, other than listening very carefully and trying to play the same thing. This is much harder than it seems at first glance! Like with many problems, though, what seems insurmountable at first can be broken down into simpler problems.

The scale of a song is more property called the key, the set of sharps or flats that are used by default when playing the song. It will usually have the same name as the first and the last chord played (although there are exceptions). As you've already guessed, the key will definitely help you determine notes of the melody! But note, that songs will usually also contain some notes outside of the scale.

If you've got a start, and are trying to figure out the rest, it can help to train specifically on accurately guessing intervals, i.e. how many notes higher or lower. Here are someexercises for that purpose; getting better is simply a matter of practice!

It may seem more difficult, but having at least a few of the song's chords will also help you find notes in the melody. The other answers have a lot of useful advice on this! Often, notes in the melody will be found in the chord that is currently being played, especially at the beginning of a phrase (although, like everything, there are exceptions). Of course, the vice versa is also true, and if you have some of the notes of the melody, you can use those to help you find the chords! And just like a jigsaw puzzle, the more pieces you have, the easier it is to place others.

I'd recommend starting with something dead simple, even a nursery song, that you don't already have sheet music for. Seriously, stuff like Twinkle Twinkle Little Star. It may seem silly, but these songs will usually have fewer left-field accidentals and tricky modulations.

You've jumped into the deep end of the pool - try to pick up as much musical theory as you can to bolster your understanding. Best of luck!

Now we know you're looking for chords, the answer is a little simpler.

To find the key chord, often it's the chord that starts the main verse, sometimes the chorus, and usually the last chord.Not easy if the tune fades at the end !

The bass part gives a great clue, so isolate the chord that you think is the important one mentioned above. Listen to the bass note, and find it on your 6th string. There's a very good chance that an E shaped chord on that fret will give the chord you're looking for.

Most songs will be diatonic, and use some chords from the main 6 found in that key. They will be -3 majors, I, IV and V, and 3 minors - ii, iii and vi. Namely, in C - C, F and G, Dm, Em and Am. When you've established the first chord,listen to where it changes into the next. If that one is major, then you've a 50/50 chance of guessing what it will be. E.g. 1st chord C maj. next, if also maj., will be either F or G. If the chord is minor, then the odds are 33% , as in Dm, Em or Am. This will cover 90% or more of the songs you need to transcribe, I hope, among the pop music of the last 50 or so years.

Obviously, more complex songs will use other chords, but, hey, we all have to start somewhere simple.

Generally the first chord in a piece will start on either the tonic or the dominant. SO you will get a strong idea there. You will also look for any accedintals which may indicate whether the piece is in a minor key and most importantly you will look at the key signature which is there to give an indication of key.